Nonaqueous-electrolyte secondary batteries, such as lithium-ion secondary batteries, are secondary batteries having high charge and discharge capacities and enabling high powers to output. At present, lithium-ion secondary batteries have been used mainly as a power source for portable electronic devices. In addition, lithium-ion secondary batteries are expected to serve as a power source for electric automobiles having been anticipated to prevail from now on. Consequently, a lithium-ion secondary battery with a much higher capacity has been desired. The selection and designing of negative-electrode active materials have been investigated in order to obtain high-capacity lithium-ion secondary batteries.
In recent years, as a negative-electrode active material for lithium-ion secondary battery, materials including an element being able to alloy with lithium, such as Si and Sn possessing charge and discharge capacities exceeding the theoretical capacity of carbonaceous material greatly, have been investigated. For example, as a negative-electrode active material, SiOx (where 0<“x”<2) with a high capacity and superior cyclability has been proposed. SiOx (where 0<“x”<2) is a general formula expressing a generic name for amorphous silicon oxide obtainable by using silicon dioxide (SiO2) and metallic silicon (Si) as the raw materials. When SiOx is heat treated, the SiOx is separated into two phases, an Si phase and an SiO2 phase, by the internal reaction of solid. The Si phase obtainable by the separation is very fine, and the Si phase is dispersed inside the SiO2 phase. Of Li being inserted into SiOx during initial charging, Li being inserted into the SiO2 phase is less likely to be eliminated at the time of discharging. Consequently, SiOx has a large so-called irreversible capacity. Hence, in order to decrease the irreversible capacity, using a silicon oxide containing lithium, or a silicate containing lithium, as a negative-electrode active material has been proposed. For example, in Patent Application Publication No. 1 (i.e., Japanese Unexamined Patent Publication (KOKAI) Gazette No. 6-325765) proposes a silicon oxide containing lithium (i.e., LixSiOy) as a negative-electrode active material. Patent Application Publication No. 1 sets forth the following: the silicon oxide containing lithium contains lithium inside the crystal structure of the oxide of silicon or within the amorphous structure; and the silicon oxide containing lithium is able to occlude and release (or sorb and desorb) lithium ions by electrochemical reactions in a nonaqueous electrolyte.
Patent Application Publication No. 1 exemplifies, as the silicon oxide containing lithium (i.e., LixSiOy), various lithium silicates, and also sets forth the following as well: using the lithium silicates as a negative-electrode active material results in enabling the irreversible capacity of Li to reduce. However, when the lithium silicates are used as a negative-electrode active material, eliminating lithium ions from the negative-electrode active material and inserting lithium ions into the negative-electrode active material at migration rates required for lithium-ion secondary battery is difficult, because the lithium silicates have an extremely small electron-conducting property.
Investigations for coating carbon, one of electrically-conductive materials, on LixSiOy have been carried out. Patent Application Publication No. 2 (i.e., Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2007-294423) proposes a silicon/silicon oxide/lithium-based composite turned conductive electrically by coating the surface of silicon/silicon oxide/lithium-based composite particles with carbon in a coating amount of from 5% by mass to 50% by mass. Patent Application Publication No. 2 sets forth the following: carbon is coated onto a silicon/silicon oxide/lithium-based composite by a thermal CVD method; and the carbon has a graphite structure exhibiting a peak at around 1,580 cm−1 in a Raman spectrum. However, Patent Application Publication No. 2 does not set forth any example in which carbon is coated on a silicon/silicon oxide/lithium-based composite.
Moreover, Patent Application Publication No. 3 (i.e., Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2011-222153) proposes a negative-electrode material comprising a silicon/silicon oxide-based composite, and a carbon film coated on a surface of the silicon/silicon oxide-based composite, wherein at least the silicon/silicon oxide-based composite is doped with lithium and an SiC peak is less in X-ray diffraction. Patent Application Publication No. 3 sets forth a method in which carbon coating is carried out to the silicon/silicon oxide-based composite at a temperature of 1,100° C. or more by a thermal CVD method followed by doping the silicon/silicon oxide-based composite with lithium.
Thus, no lithium silicate coated by an electrically-conductive material has not been found in related art.